Drilling in deep waters involves other and more complex challenges compared to drilling in shallow waters. This is also the case for risers to be used in such deep waters, which must withstand potentially higher tension loads and pressures etc. The vessel's heave motion may also coincide, or nearly coincide, with the riser string's natural frequency, resulting in even higher tension loads. This is a challenge in situations where the riser string has a free end, i.e., when the riser string is not connected the sea bottom. Such situations may occur when using the riser for hoisting or lowering of equipment, for example, a blow out preventer (BOP) from the surface to the sea bottom, or when the riser has been disconnected from a BOP or manifold. On drilling vessels, a critical load condition is hoisting/lowering of the BOP, or during other heavy lifts, using risers. No heave compensation systems are connected in this load condition, so that the heavy load (BOP) will try to follow the vessel's heave motion. The length of the risers with BOP depends on the water depth, but could exceed 3000 meters. The riser connection to the vessel is stiff and is either connected to the DDM (Derrick Drilling Machine) or on the spider or other hang-off plate on a deck (drill floor). Due to this hard hang-off system, the vertical naturally frequency of the riser string could meet the frequency of the heave motion on the floating vessel, potentially resulting in a considerable dynamic load amplification.
GB 2294713 A describes a deep water riser string that has a central tube, peripheral lines, and a base located at the lower end of the central tube. The central tube is fitted with means for retaining the peripheral lines in a position relative to the central tube. The lower ends of the peripheral lines are linked to a device arranged on the base, permitting a certain axial movement of at least one of the lines relative to the central tube. The string is fitted with damping means.